So here's something I rarely see gone into with Vinyl, probably because her adult persona is interesting for factors other than cuteness. What do we think her childhood was like?

Most of the time, if it's brought up, it seems assumed she just had a generic childhood, with two loving parents who encouraged her passion for DJ style music. However, ever since the 100th episode, a new theory has began to circulate about Vinyl and Octavia. In the episode, the two acted familiar, but not like lovers per say. More like VERY close friends, or rather, family. Vinyl could just move her head a few times and Octavia knew exactly what she wanted. If they're older this could imply full fledged long term marriage, but sisterhood seems pretty likely too, especially if they're not that old.
But here's the thing, Octavia and Vinyl have virtually NOTHING in common beyond gender and an interest in music. Coat color, mane color and style, stature, music style and even pony race. MAYBE the two came from a mixed racial union, but even then you would expect them to share SOME traits.

My best guess, if they are family, one is adopted. I have no reason to believe it's not Octavia who was adopted, but I always have it be Vinyl instead. In my stories, she and Octavia are actually pretty close, Octavia being one of the ponies who helped her, once helping her get adjusted to a new family, and helping essentially rescue her from orphandom/foster care, and further sticking by her when she lost her voice. Not every close relationship has to turn sexual, and I always paint this two as probably CLOSER than most couples.

You can read my rendition of her childhood in "Our Stories" here:
https://www.fimfiction.net/story/384608/2/our-stories/vinyl-scratch-i-love-my-family More on that might be coming, we'll see.

What are everyone else's theories on how Vinyl grew up?

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That's the beauty of background ponies. They are an empty canvas waiting to be filled. We honestly don't anything as a fact about Vinyl and Octavia except that they somehow both live in Ponyvile. Also that Octavia preforms in Canterlot and Vinyl preforms in Manehatten. But well when it comes to background ponies time and space are barely a barrier. And well they probably are in platonic relationship but we all know that good part of fandom prefers romantic relationship. I have seen them being sisters before, even twins. Or they don't share both parents. Hmm don't think I have seen adoption so bonus points for you there.

As for my guess? I prefer that Vinyl is from Ponyville and yeah with two parents because why not. But she studied in Canterlot which is where she meet Octavia. I see Vinyl as being younger one and she was quite a rebel in school and liked to hang out more with the older ponies (like Lyra and others but not really part of their friendship group). She met Octavia when Vinyl was punished by her music theory teacher to listen to some orchestra. Octavia would be the one actually approaching the unusual pony in the crowd because despite Octavia's appearance she is bored of the Canterlot nobility and likes more honest ponies. She would ask Vinyl what she thought of the performance and Vinyl would try not offend Octavia to which Octavia would respond by pointing out that orchestra really was bad and that Vinyl shouldn't pretend because of her. Not the most beautiful of friendship beginnings but it's a start. And they would hang out much through Vinyls stay in Canterlot.

I see Octavia moving to Ponyvile probably when Rarity went crazy and zapped those Mexican ponies and replaced them with Octavia and her ensemble. Not having any place to stay she reunited with Vinyl and they continued their friendship. Octavia grew to like Ponyville and chose to move in while still working in Canterlot (hey Rarity can do it and other unicorns seem to travel daily too). And no question that Octavia and Vinyl would be on same wave length and probably would have more in common than just both being musicians.

Also I am kinda toying with the idea that Vinyl is actually Rarity's cousin. Mostly because they are both white unicorns but also because Rarity somehow got Vinyl to play music during Suited for Success episode. I don't know much about DJs but I don't think they usually preform in fashion shows but I can see Vinyl doing a favor for a cuz. Also Rarity just happened to find available place in Manehatten right under Vinyl's studio. Bet they would have hated each other when they were kids but would mellow out as adults. Or at least Rarity would be the one being all uptight while Vinyl would find her amusing and easy to tease.

I don't yet have any fic about any of this but hey figured maybe some ghost would be interested in my thoughts.